Cython & C++: passing by reference

Question

I am a noob with Cython and C++, so I have a question on argument passing. I want to avoid passing a copy of an argument in the following scenario:

# somefile.pyx
#distutils: language = c++
from libcpp.vector cimport vector

def add_one(vector[int] vect):
    cdef int i
    n = vect.size()
    for i in range(n):
        vect[i] += 1

cdef vector[int] v
for i in range(100000):
    v.push_back(i)
add_one(v) # <-- ??

I want the method add_one to just modify v "in-place." I believe in C++, you can achieve this by pre-pending the argument with &, which means that any changes to the pointer is passed to the pointee. That way, you don't have to worry about passing a pointer or the actual object, i.e.

add_one(v); # in c++

Can I do the same in Cython, or do I have to explicitly change the arg type to a reference instead, i.e. def add_one(vector[int]* vect)?

Somewhat related question on [cython-users](https://groups.google.com/forum/#!forum/cython-users): ["Question about pass-by-reference"](https://groups.google.com/forum/#!topic/cython-users/6c2ixr4xEZA) — iljau, Feb 12 '14 at 07:51

score 24 · Accepted Answer · edited Jul 20 '17 at 12:01

24

Found the answer to my own question. Apparently, you can pass by reference, but the function MUST be cdef'ed, not def'ed. i.e.

# somefile.pyx
#distutils: language = c++
from libcpp.vector cimport vector

cdef void add_one(vector[int]& vect):
    cdef int i
    n = vect.size()
    for i in range(<int>n):
        vect[i] += 1

cdef vector[int] v
for i in range(100000):
    v.push_back(i)
add_one(v)

edited Jul 20 '17 at 12:01

Eli Korvigo

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answered Feb 12 '14 at 07:49

richizy

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i do pass-by-ref for std::string but can't change its value inside the function – Dee Nov 18 '21 at 09:57

Cython & C++: passing by reference

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